In order to accurately control a rotary machine having a rotor provided with a permanent magnet, such as a synchronous electric motor or an induction synchronous electric motor, information about the rotor position of the rotary machine and current flowing in the rotary machine is needed. The rotor position information is conventionally obtained by attaching a position sensor or the like on the rotary machine, but in view of cost reduction, space saving, and reliability improvement, a position sensorless method is being employed to obtain the rotor position information.
Examples of such a position sensorless control method for rotary machine are a position sensorless control method using induced voltage of a rotary machine as shown in Patent Document 1 and a position sensorless control method using saliency as shown in Patent Document 2. In the position sensorless control using induced voltage, when the speed is zero, the induced voltage is also zero, and therefore the rotor position cannot be estimated properly. In addition, in the position sensorless control using saliency, since the saliency used for rotor position estimation changes at two times the frequency of the rotor position, the frequency of the estimated position is also two times that of the rotor position. That is, the estimated position takes the same value between a range of 0 to 180 degrees and a range of 180 to 360 degrees of the rotor position of the rotary machine, and therefore this method is not sufficient for reliable detection of the rotor position.
Considering the above, at least when a rotary machine is started from a speed close to zero, besides the position sensorless control method using saliency, another method for estimating rotor position information about the rotary machine is needed. As such a method, for example, a method using magnetic saturation of the rotary machine as shown in Patent Document 3 is known.
In this method, 2n (n is a phase number and is a natural number equal to or greater than 3) voltage vectors having the same amplitude and having phases separated at equal intervals are applied to a rotary machine, and then a rotor position is detected from a summed current value obtained by summing detected values of currents flowing when a pair of such voltage vectors having phases different by 180 degrees from each other are applied. This method requires a precondition that the rotary machine becomes a magnetically saturated state by application of the voltage vectors.
For example, Patent Document 4 discloses an adjustment method for ensuring the magnetically saturated state. In this method, a pulse voltage value is gradually increased so that differential current ΔIb between the maximum value Imax of the H-level value of current when pulse voltage is applied, and the H-level value of current corresponding to pulse voltage having a phase different by 180 degrees from Imax, becomes equal to or greater than a predetermined value, thus adjusting the voltage pulse value automatically.